It is absolutely essential to conserve forests for the protection of the global atmosphere and for suppressing the greenhouse effect of CO2. A large problem is posed as to how to efficiently utilize paper resources held now to maintain new logging to a minimum and to keep the balance with the regeneration of forests including afforestation. The current reuse of paper resources is the so-called “recycle” in which paper fibers processed through the deinking step for peeling image formation materials are subjected to paper-remaking to make low-quality paper, which is used in a different application according to its purpose. However, it is pointed out that the current reuse of paper resources involves the problems concerning the high cost of the deinking step and new environmental pollution caused by the treatment of waste fluids.
As image erasable image formation materials which have been used so far, there is an example applied to a toner used in the electro-photographic printing system. This toner is obtained as an image formation material by blending a material of which the image can be erased by heat or an organic solvent with a toner binder. However, it is necessary to heat to fix the toner in this electro-photographic printing system and there is a room for contrivance in view of energy saving. Also, when an erasable image formation material is used as a toner, this gives rise to the problem that if heat is added to erase an image, dyes disappear but the resin remains and the image can be incompletely erased. This is the reason why the utilization method resistant to the residence of a resin. This remaining of the resin is the reason why the erasing method is not so popular so far.
In light of this situation, an image formation method using an ink jet system is investigated instead of the electro-photographic printing method. The ink jet printing and offset printing are different from printing using a toner in the point that they require no heating equipment used to fix the toner. If an erasable image formation material is utilized in the ink jet or offset printing, this material is expected to enable more efficient utilization of resources and to produce the effect of reducing the gas giving the greenhouse effect. The possibility of printing using a usual ink jet printer makes it easy to introduce a printer using erasable ink even for a domestic use. The reasons mentioned above very increase the necessity of the erasable ink usable in the ink jet or offset printing.
The inventors of the present invention have paid their attentions to such a phenomenon that an increase in the interaction between a chromatic material (leuco dye) and a color developer puts the chromatic material into a color developed state whereas a decrease in the interaction puts the chromatic material into a decolored state, to develop an image formation material so far. An image formation material comprising a chromatic material and a color developer can stably exhibits a color-developed state at temperatures around ambient temperature. Moreover, an image drawn using the image formation material can be sufficiently decolored only by treating plain paper by using heat or an organic solvent without adding a decoloring agent used to catch the color developer to the image formation material. Then, the decolored state can be maintained at a practical temperature for a long period of time. The inventors of the present invention have proposed an image erasing process and an image erasing device in relation to these image formation materials as reuse technologies of paper. These image formation materials keep the color developing state and decolored state of an image highly stably and the material highly safe, are applicable to all of the electrophotographic toners, liquid inks, ink ribbons and writing instruments and enable large-scale erasing treatment, showing that these image formation materials have merits that have not been obtained by conventional technologies.
Liquid erasable ink using an emulsion is conventionally known as disclosed in JP-A2002-294104 (KOKAI) (Patent Document 1). This erasable ink is used in the form of an emulsion, suspension or dispersion solution, in which basically, high-molecular microparticles are dyed with a chromatic material and a color developer and decolored by a decoloring agent. The emulsion resin described in JP-A 2002-294104 (KOKAI) is a polymer or copolymer resin of a monomer such as acrylonitrile, styrene, acrylic acid, methacrylic acid, methylacrylate or methylmethacrylate.
However, erasable inks using these resins have the drawbacks that they are considerably inferior in color developing ability and coloring ability to usual non-erasable inks. Therefore, these inks have large problems as to how to improve the color developing ability and coloring ability of the ink for colors, such as black or blue, which are less practicable if these color densities are not deep. The technologies described in Patent Document 1 did not more than put writing instruments to practical use which exhibited a pink or yellow color which might be light to accomplish the purpose satisfactorily. These inks were not adopted for ink jet and offset inks because they have deteriorated coloring ability. Moreover, the technologies described in Patent Document 1 have the problem that the coloring temperature of the emulsion is as relatively higher as 50°C.